OBJECTIVE: To define the developmental potential of marmoset ES cells in vivo, and to test the potential of marmoset ES cell nuclei to support development when transferred to enucleated oocytes. RESULTS During 1997, we tested a series of promoter-green fluorescent protein (GFP) constructs for use as lineages markers to identify marmoset ES cells in chimeras. We identified ubiquitin and b-actin promoters as providing the most consistently high levels of GFP expression in primate ES cells. FUTURE DIRECTIONS We will use these GFP expression constructs to analyze the tissue distribution of ES cell derivatives in chimeric marmoset fetuses, and thus define the developmental potential of marmoset ES cells. KEY WORDS Callithrix jacchus, chimera, embryonic stem cells, blastocyst